REFLECTION  AND  TRANSMISSION  OF  ULTRA-VIOLET 
LIGHT  BY  SODIUM  AND  POTASSIUM. 


SUBMITTED   TO   THE  BOARD    OF    UNIVERSITY  STUDIES  OF  THE  JOHNS   HOPKINS   UNIVERSITY 

IN  CONFORMITY   WITH  THE  REQUIREMENTS  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


ma,  KATH, 


Reprinted  from  the  PHYSICAL  REVIEW,  N.S..  Vol.  XV,  No.  2,  February, 


[Reprinted  from  THE  PHYSICAL  REVIEW,  N.S.,  Vol.  XV.  No.  2,  February,  igao.j 


REFLECTION  AND  TRANSMISSION  OF  ULTRA-VIOLET 
LIGHT  BY  SODIUM  AND  POTASSIUM. 

BY  MABEL  KATHERINE  FREHAFER. 
'» 

SYNOPSIS. 

Reflecting  power  of  K  and  Na,  for  wavelengths  from  0.25  to  0.55/1.  (i)  Normal 
incidence  (10°).  Opaque  mirrors  in  contact  with  fused  quartz  were  tested  with  mono- 
chromatic light  from  a  mercury  arc.  A  sodium  photoelectric  cell  was  used  to  com- 
pare the  intensities  of  incident  and  reflected  beams.  It  was  found  that  sodium  main- 
tains a  remarkably  high  reflecting  power  throughout  the  ultra-violet,  increasing 
slightly  from  a  flat  minimum  near  36ooA  to  80  per  cent,  at  2536A.  The  reflecting 
power  of  potassium  on  the  contrary  decreases  rapidly  with  the  wavelength,  reaching 
the  very  low  value  of  n  per  cent,  at  2536A.  A  thin  film  of  potassium  deposited  and 
kept  at  —190°  C.  gave  a  curve  of  the  same  form.  Among  the  metals  so  far  investi- 
gated in  the  ultra-violet,  therefore,  sodium  and  potassium  have  respectively  the 
highest  and  lowest  reflecting  powers.  (2)  Polarized  light  at  45°  incidence.  The  ratio 
of  the  reflecting  powers  for  light  polarized  with  the  electric  vector  respectively  parallel 
and  perpendicular  to  the  plane  of  incidence,  was  found  for  both  sodium  and  potas- 
sium to  have  a  maximum  value  near  36soA.  There  is  also  a  common  minimum  at 
about  334iA  which  is  far  more  pronounced  in  the  case  of  sodium. 

Transmitting  power  ofK  and  Na  films  for  wave-lengths  from  0.25  to  0.55  p,.  Films  de- 
posited on  quartz  and  on  glass  at  liquid-air  temperatures  were  studied  while  still  cold 
by  the  use  of  a  special  apparatus  and  compared  with  the  ordinary  colloidal  films 
prepared  at  room  temperature.  The  results  are  shown  graphically.  Under  com- 
parable conditions,  the  transmitting  power  of  potassium  is  considerably  greater  than 
that  of  sodium. 

Thin  films  of  K  and  Na  deposited  at  —190°  C.  on  quartz  have  a  metallic  appearance; 
on  being  warmed  a  thin  film  becomes  peppered  with  holes  and  an  ooaaue  film  loses 
its  luster. 

CONTENTS. 

Introduction. 

Statement  of  the  Problem.     Historical. 

Preparation  of  the  Mirrors  and  Means  of  Support  for  (i): 

1.  Massive  Mirrors. 

2.  Films  Deposited  at  Low  Temperatures. 
Measurement  of  Intensity. 

I.  Normal  Incidence. 

Arrangement  of  Apparatus  and  Experimental  Procedure  with  Mirrors  ,.  i 
Computation  and  Results. 

II.  Oblique  Incidence.     (Polarized  Light). 

Arrangement  of  Apparatus.     Discussion  of  Difficulties.     Results. 
Films  Deposited  at  Low  Temperature. 

1.  For  Reflection:  Apparatus  and  Result. 

2.  For  Transmission:  Apparatus  and  Results. 
Summary. 


500874 


*: ;  •;  ••;  .*, •  «. 

\;  :•":.*';:  ;  ••      '.  •**-     : 

III  MABEL  K.  FREHAFER. 


A  STUDY  of  the  reflecting  power  of  the  alkali  metals  in  the  ultra- 
^~~*-  violet  is  of  special  interest  in  connection  with  the  selective  photo- 
electric effect.  Since '  this  property  of  metals  probably  points  to  a 
resonance  phenomenon,  it  seems  possible  that  a  knowledge  of  the  optical 
properties  may  throw  some  light  on  the  mechanism  of  this  effect.  The 
relation  between  the  electron  emission  and  the  light  absorption  in  the 
selective  effect  does  not  appear  to  be  a  direct  linear  one,  such  as  seems  to 
exist  in  the  normal  effect  for  a  given  wave-length. 

The  alkali  metals,  especially  sodium  and  potassium,  are  highly  re- 
flecting in  the  visible  spectrum,  with  large  extinction  coefficients  and 
extremely  small  refractive  indices.  Drude's1  values  of  the  index  of 
refraction  of  sodium  and  of  the  extinction  coefficient,  obtained  by 
analyzing  the  state  of  polarization  of  yellow  light  reflected  from  the 
surface  of  the  molten  metal,  were  the  only  data  available  until  the 
work  of  R.  W.  and  R.  C.  Duncan2  who,  using  Drude's  method,  deter- 
mined the  optical  properties  of  both  sodium  and  potassium  as  a  function 
of  the  wave-length  in  the  visible  region.  The  metals  were  used  in  the 
solid  form  backed  by  glass.  The  first  direct  method  for  the  determina- 
tion of  the  reflecting  powers  of  these  metals  was  used  by  Nathanson3 
for  sodium,  potassium  and  rubidium,  with  white  light  and  monochromatic 
polarized  light  for  the  region  6409-4546  A.  He  also  used  the  solid 
metal  backed  by  glass.  The  intensity  of  the  light  was  measured  by 
means  of  a  photoelectric  cell,  connected  to  an  electrometer. 

The  well-known  "  selective  effect  "  of  the  alkali  metals  and  a  few 
other  metals  offers  a  particularly  promising  field  of  research  for  the 
relations  existing  between  the  ejected  electrons  and  the  atoms.  It  was 
with  the  hope  of  throwing  some  light  on  this  subject  that  this  investi- 
gation of  the  reflecting  powers  of  sodium  and  potassium  through  the 
regions  in  which  the  selective  effect  occurs  has  been  attempted.  Elster 
and  Geitel4  were  the  first  to  show  that  the  selective  effect  is  directly 
concerned  with  the  position  which  the  electric  vector  in  the  incident 
light  has  relative  to  the  metallic  surface;  that  is,  the  photoelectric 
current  is  a  maximum  when  the  plane  of  polarization  is  perpendicular  to 
the  plane  of  incidence  (i.e.,  the  electric  vector  is  in  the  plane  of  incidence) , 
and  is  a  minimum  when  these  planes  are  parallel  (i.e.,  the  electric  vector 
is  at  right  angles  to  the  plane  of  incidence).  Pohl  and  Pringsheim5  have 

1  Drude,  Annalen  der  Physik,  64,  159,  1898. 

*  R.  W.  and  R.  C.  Duncan,  PHYSICAL  REVIEW,  36,  294,  1913. 

8  Nathanson,  Astrophysical  Journal,  44,  137,  1916. 

4  Elster  and  Geitel,  Annalen  der  Physik,  52,  540,  1894;  55,  684,  1895;  61,  445,  1897. 

6  Pohl,  Physikalische  Zeitschrift,  10,  542,  1909.  Pohl,  Deutsche  Physikalische  Gesell- 
shaft,  ii,  339  and  609,  1909.  Pohl  and  Pringsheim,  Deutsche  Physikalische  Gesellshaft,  12, 
215  and  349,  1910. 


VOL.  XV. 
No.  2. 


ULTRA-VIOLET  LIGHT. 


112 


determined  the  photoelectric  current  as  a  function  of  the  wave-length, 
both  for  E\\  and  E±,  as  well  as  for  unpolarized  light  incident  at  some 
angle  different  from  zero  (corresponding  to  E\\).  These  results  have 
been  embodied  in  the  familiar  diagram  reproduced  here. 


200/L/x 


WAVE.  LENGTH. 
Fig.  1. 


The  ordinates  represent  the  amount  of  electron  emission  for  equal 
amounts  of  energy  in  the  exciting  light. 

The  ratio  E\\/E±  gives  a  curve  similar  to  E\\  and  yields  the  same  value 
of  X  for  the  maximum  of  the  selective  effect. 

The  conclusion  from  their  work  and  that  of  others  is  that  when  the 
electric  vector  in  the  incident  light  has  a  component  perpendicular  to  the 
metal  surface  illuminated,  the  resulting  electron  emission,  called  the 
selective  effect,  increases  enormously  for  a  number  of  elements,  the 
maximum  values  occurring  at  definite  values  of  X.  For  sodium  and 
potassium  these  values  are  as  follows: 

Na:     X  =  340  ju/z  (Pohl  and  Pringsheim)1  . 

360  nn  (Richardson  and  Compton)2 
K:  436  nfj.  (Pohl  and  Pringsheim)3 

440  ju/i  (Braun)4 

A  study  of  the  velocities  of  the  electrons  emitted  in  the  normal  and 
selective  effects  was  made  by  Hughes,4  but  these  values  proved  to  be 
only  slightly  different  from  each  other.  The  difference  in  the  mechanism 
of  the  normal  and  selective  effects  is  still  left  undetermined — beyond  the 
fact  that  the  former  suggests  an  action  of  free  electrons,  the  latter  of 
electrons  in  the  atoms,  of  frequencies  characteristic  of  the  metal. 

The  work  of  Richardson  and  Compton7  has  shown  a  maximum  photo- 

1  Pohl  and  Pringsheim,  Deutsche  Physikalische  Gesellshaft,  14,  49,  1912. 

2  Richardson  and  Compton,  Philosophical  Magazine,  26,  549,  1913. 

3  Braun,  Dissertation,  Bonn,  1906. 

4  Hughes,  Philosophical  Magazine,  31,  100,  1916. 


MABEL  K.  FREHAFER. 


[SECOND 

[SERIES. 


electric  current  to  exist  even  for  the  normal  effect,  that  for  sodium  for 
instance  occurring  at  227  juju.  The  photoelectric  activity  of  metals  is 
evidently  a  complicated  phenomenon.  Investigation  of  the  optical 
properties  of  metals  for  wave-lengths  covering  these  "  normal  "  maxima 
would  also  be  of  interest. 

PREPARATION  OF  THE  MIRRORS,  AND  MEANS  OF  SUPPORT. 
The  metal  surfaces  used  were  of  two  kinds:  those  of  the  solid  metal 
in  a  lump  against  quartz,  such  as  had  been  used  by  R.  W.  and  R.  C. 
Duncan  and  by  Nathanson;  and  thin  films  of  the  metal  deposited  on 
quartz  by  evaporation  at  the  temperature  of  liquid  air.  The  arrange- 
ment for  making  mirrors  of  the  first  kind  is  shown  in  Fig.  2.  Pieces  of 


Pump 


Fig.  2. 

sodium  or  potassium  are  placed  in  a  small  tube  and  slipped  into  the 
end  A,  which  is  then  sealed  off.  The  apparatus  is  heated  locally  with 
a  Bunsen  flame  to  drive  off  water  vapor  and  gases,  particularly  from  the 
plate  Q  which  is  of  fused  quartz.  Crystalline  quartz  is  very  likely  to 
crack  when  the  hot  metal  is  poured  over.  The  pump  is  kept  in  action 
until  the  pressure  has  become  fairly  low  and  perfectly  steady.  Parts 
A  and  B  are  now  placed  inside  a  can  and  heated  slowly,  and  the  gas 
evolved  is  pumped  off.  In  time  the  metal  distils  over  into  C,  where  it 
collects  in  a  small  pool.  After  the  apparatus  is  sealed  off  from  the  pump 
the  molten  metal  is  poured  over  into  D  and  brought  into  contact  with 
the  quartz  plate.  If  the  cooling  of  the  metal  takes  place  slowly  and 
uniformly,  and  if  the  quartz  is  clean,  the  metal  shows  little  tendency  to 
withdraw  from  the  quartz  surface  and  the  resulting  mirror  presents  a 
fairly  large  area  free  from  blemishes.  Mirrors  that  are  dotted  over  with 
small  holes  result  from  too  rapid  or  non-uniform  cooling.  Bulb  D  is 
cracked  open,  the  quartz  plate  with  the  metal  adhering  to  it  is  removed, 
and  "  half  and  half  "  is  quickly  applied  until  the  metal  is  made  air- 
tight. Mirrors  of  sodium  and  potassium  made  in  this  way  kept  very 
well  during  many  weeks,  apparently  without  any  deterioration.  During 
the  damp  days  of  spring  the  metals  showed  a  tendency  to  oxidize  around 
the  edges,  hence  special  precautions  are  necessary  to  preserve  them. 


VOL.  XV. 
No.  2. 


ULTRA-VIOLET   LIGHT. 


114 


For  purposes  of  support,  a  glass  plate  is  sealed  on  parallel  with  Q. 
The  mirror  is  supported  vertically  (see  Fig.  3)  against  a  brass  plate,  held 
rigidly,  in  which  had  been  drilled  a  hole  5  mm.  in  diameter. 

The  mirror  was  pressed  against  three  pins  in  the  brass  plate,  by  a 
tight  spring  surrounding  a  brass  rod  that  touched  the  glass  plate.  With 
this  arrangement  the  mirrors  could  be  replaced  with  great  accuracy  in 
the  same  position  relative  to  the  hole,  which  was  very  essential  for 


\5uartz 


Fig.  3. 

readings  that  could  be  reproduced.  The  front  side  of  the  brass  support 
was  blackened  with  soot.  The  light  was  incident  on  the  mirror  at  an 
angle  of  10°. 

The  making  of  metallic  films  of  sodium  and  potassium  by  cooling  the 
glass  or  quartz  surface  to  the  temperature  of  liquid  air  was  suggested 
by  Prof.  R.  W.  Wood.1  A  small  amount  of  metal  is  distilled  over  on  to 
the  surface  of  a  bulb,  similarly  to  the  method  already  described  (see 
Fig.  2),  except  that  A  is  heated  locally,  driving  the  metal  to  B,  and  is 
then  sealed  off  from  the  apparatus.  B  is  then  heated  sending  the  metal 
to  C,  and  is  also  sealed  off.  Two  distillations  are  usually  sufficient. 
The  quartz  plate  is  now  placed  in  C.  The  apparatus  is  now  removed 
from  the  pump,  and  bulb  D,  of  various  shapes  and  sizes  according  to 
the  nature  of  the  experiment,  is  cooled  down  to  —  190°  C,  as  is  also 
the  lower  part  of  C,  containing  Q.  A  small  flame  is  then  applied  to  the 
upper  part  of  C,  thus  vaporizing  the  metal,  some  of  which  deposits  on 
the  cold  quartz  plate  below.  When  conditions  are  favorable  the  resulting 
film  has  a  truly  metallic  appearance,  quite  different  from  the  colloidal 
film  formed  at  ordinary  temperatures.  On  warming  up  to  room  tempera- 
ture, a  thin  film  becomes  peppered  with  holes  and  appears  to  evaporate; 
an  opaque  film  loses  its  luster. 

1  R.  W.  Wood,  Philosophical  Magazine,  38,  98,  1919. 


115  MABEL  K.  FREHAFER. 

MEASUREMENT  OF  INTENSITY. 

As  in  Hulburt's1  and  in  Nathanson's3  work,  the  intensity  of  the  reflected 
light  was  measured  directly  by  means  of  a  photoelectric  cell.  This  was 
a  bulb  coated  with  sodium,  with  a  short  tube  about  I  cm.  in  diameter 
carrying  a  fluorite  window.  This  cell,  which  was  used  by  Dr.  E.  O. 
Hulburt,  in  his  research  on  reflecting  power  of  metals,  obeys  the  law  of 
direct  proportionality  between  the  light  intensity  and  the  photoelectric 
current,  through  a  range  of  intensities  far  exceeding  those  used  in  the 
present  work.  This  was  tested  by  varying  the  distance  between  the 
cell  and  a  tungsten  lamp.  A  later  test  for  monochromatic  light  (4358  A. 
and  3130  A.)  gave  the  same  result.  Except  at  the  anode  and  window  the 
cell  was  covered  with  tinfoil,  which  served  very  well  to  remove  charges 
from  the  surface  of  the  glass.  It  was  found  advisable  to  keep  the  cell 
in  a  box  in  order  to  make  the  "  dark  leak  "  small.  The  anode,  which 
was  connected  to  a  sensitive  electroscope,  was  charged  to  no  volts; 
the  cathode  and  case  of  electroscope  were  earthed.  In  the  winter  the 
dark  leak  took  20  to  30  minutes,  while  many  of  the  actual  readings  for 
the  intensity  of  the  reflected  light  occupied  10  to  20  seconds.  In  the 
spring  the  dark  leak  was  greater.  It  is  measured  by  taking  the  rate  of 
motion  of  the  gold  leaf  when  the  mirror  is  removed  from  the  brass  support 
(see  Fig.  4  below). 

NORMAL  INCIDENCE. 

Arrangement  of  Apparatus.— This  is  shown  in  the  following  diagram. 
The  apparatus  was  set  up  in  a  dark  room,  and  suitable  screens  were 
used  to  protect  the  mirror  M  from  all  light  except  that  passing  through  52. 
The  source  of  light  was  a  mercury  arc  in  quartz,  designed  by  Dr.  A.  H. 
s, 


Fig.  4. 

Pfund,  and  run  on  storage  batteries  at  120  volts.  This  lamp  runs  very 
steadily  for  hours,  giving  light  of  constant  intensity.  The  light  is 
passed  through  a  quartz  spectrograph  so  designed  (by  Dr.  Pfund2)  that 
the  different  wave-lengths  were  brought  into  focus  at  the  slit  by  a  single 
rack  and  pinion  adjustment.  The  first  slit  (towards  the  arc)  was  I  mm. 
in  width,  the  second  slit  was  .6  mm.;  these  were  kept  the  same  through- 
out. The  intensities  of  the  lines  in  the  arc  differ  greatly.  The  more 
intense  lines  were  cut  down  by  the  use  of  suitable  screens  in  front  of  5i ; 
narrowing  Si  failed  to  give  reproducible  results. 

1  Hulburt,  Astrophysical  Journal,  42,  205,  1915. 
*  Pfund,  PHYSICAL  REVIEW,  7,  29,  1916. 


NoL2XV']  ULTRA-VIOLET  LIGHT.  I  1  6 

Since  the  reflecting  power  of  mercury  against  quartz  has  been  deter- 
mined with  accuracy  by  Hulburt1  in  the  ultra-violet,  using  a  direct 
method  with  a  photoelectric  cell,  and  by  Meier2  in  the  visible  spectrum 
and  partly  in  the  ultra-violet  using  the  katoptric  method,  a  comparison 
method  with  mercury  was  decided  upon  for  the  present  investigation. 
The  procedure  is  to  place  the  mirrors  in  succession  —  say  mercury, 
sodium,  potassium,  mercury  —  in  the  brass  support  at  M,  reading  for 
each  the  rate  of  fall  of  the  gold  leaf.  The  second  setting  of  the  mercury 
mirror  serves  as  a  check  on  the  constancy  of  the  incident  light.  The 
readings  obtained  for  a  given  set  of  mirrors  for  different  sets  of  observa- 
tions differed  by  not  more  than  6  per  cent. 

COMPUTATION13  AND   RESULTS. 

I  f  r  is  the  reflecting  power  of  quartz, 
Rq  is  the  reflecting  power  of  sodium  or  potassium  in  contact  with 

quartz, 

R'  is  the  reflecting  power  of  the  whole  mercury  mirror, 
i   is  the  photoelectric  current  due  to  the  light  reflected  by  the  whole 

sodium  or  potassium  mirror, 
V  is  the  photoelectric  current  due  to  the  light  reflected  by  the  whole 

mercury  mirror, 
then,  to  a  fair  approximation, 

'"'    ~  ' 


f  n  —  i  \2 

The  value  of  r  is  computed  from  Fresnel's  formula  I  —  -j—  -  )  ;  the  appro- 

\n  +  I  / 

priate  values  of  n,  the  refractive  index  of  quartz,  are  interpolated  from 
those  determined  by  Martens.15  The  values  of  i  and  i'  are  inversely 
proportional  to  the  rate  of  leak  of  the  electroscope  (corrected  when 
necessary  for  the  dark  leak). 

Some  of  the  results  are  shown  in  Table  L,  where  iNa/i'Ha  'ls  the  ratio 
of  the  intensities  of  the  reflected  light  from  the  whole  sodium  mirror 
and  the  whole  mercury  mirror  respectively.  The  number  of  seconds 
given  in  parentheses  is  the  original  reading,  not  corrected  for  the  dark 
leak.  The  corrected  reading  is  the  one  directly  below.  Rq  is  the  re- 
flecting power  of  the  metal  in  contact  with  quartz.  The  reflecting  power 
of  the  metal  alone,  found  by  Ingersoll's1  method,  is  only  a  few  per  cent. 

1  Hulburt,  Astrophysical  Journal,  46,  i,  1917. 
3  Meier,  Annalen  der  Physik,  31,  1017,  1910. 

3  Martens,  Annalen  der  Physik,  6,  603,  1911. 

4  Ingersoll,  PHYSICAL  REVIEW,  29,  392,  1903. 


MABEL  K.  FREHAFER. 


[SECOND 
[SERIES. 


higher.  The  results  given  here  are  not  of  sufficient  accuracy  to  warrant 
its  use.  Rqf  is  the  value  Rq  (for  potassium)  assumes  when  multiplied 
by  a  suitable  factor.  This  factor  is  obtained  by  comparing  the  value 
of  Rq  for  X  5461  A.  with  that  given  by  Nathanson3  for  normal  incidence. 
The  graphs  shown  in  Fig.  5  represent  the  average  values  of  Rq  for  the 
two  metals  taken  from  many  sets  of  observations.  It  is  interesting 


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Fig.  5. 

that  the  reflecting  power  of  sodium  shows  a  minimum  in  the  region  in 
which  the  maximum  of  the  selective  effect  occurs.  Although  the  angle 
of  incidence  was  small,  the  electric  vector  had  a  small  component  per- 
pendicular to  the  surface  of  the  mirror.  Since  a  greater  energy  of  emis- 
sion in  the  form  of  electrons  suggests  a  greater  absorption  of  light  for 
this  wave-length,  a  diminution  in  the  reflecting  power  of  sodium  might 
be  expected.  A  second  point  worthy  of  notice  is  the  high  reflecting 
power  of  sodium  in  the  ultra-violet — nearly  80  per  cent. ;  this  is  higher 
than  for  any  other  substance  that  has  been  investigated. 

The  curve  for  potassium  on  the  other  hand  shows  a  rapid  decrease  in 
the  reflecting  power  with  wave-length,  the  decrease  setting  in  rapidly 
below  4358  A.  the  wave-length  at  which  the  maximum  of  the  selective 
effect  for  potassium  occurs.  The  abrupt  decrease  at  3130  A.  and  slight 
decrease  at  2804  A.  may  possibly  mean  that  a  maximum  photoelectric 
current  for  the  normal  effect  is  to  be  expected  in  the  vicinity  of  3130  A. 


VOL.  XV.l 

No.  2. 


ULTRA-VIOLET   LIGHT. 

TABLE  I. 


Reflection  from  Sodium. 

Reflection  from  Potassium. 

Wave- 
length. 

No.  of  Seconds. 

$Bfc 

A". 

A-,. 

No.  of  Seconds. 

*K 

AV 

mf 

Hg. 

Na. 

Hg. 

K. 

5461 

21.2 

15.2 

1.40 

66 

96.0 

21.2 

19.4 

1.10 

73.4 

92.5 

21.4 

15.2 

21.4 

19.4 

(35.6) 

(26.0) 

1.39 

(35.6) 

(33.7) 

1.06 

37.1 

26.8 

37.1 

35.1 

4358 

14.2 

10.6 

1.35 

62 

87.5 

14.2 

12.6 

1.12 

69.7 

88.0 

13.8 

10.2 

13.8 

12.4 

13.0 

9.5 

1.37 

13.0 

12.0 

1.08 

4047 

14.2 

10.5 

1.35 

57 

80.5 

14.2 

12.9 

1.10 

62.7 

79.0 

13.2 

9.6 

1.38 

13.2 

12.3 

1.07 

3650 

20.2 

14.0 

1.40 

55 

78.5 

20.2 

18.6 

1.08 

59.5 

74.0 

20.4 

14.8 

20.4 

18.8 

15.4 

11.2 

1.37 

15.4 

14.4 

1.06 

14.8 

10.8 

.  14.8 

14.2 

15.2 

15.2 

3341 

13.8 

10.0 

1.38 

55 

78.0 

13.8 

16.0 

.86 

46.0 

58.0 

13.6 

9.8 

13.6 

16.0 

15.2 

10.6 

1.37 

15.2 

18.0 

.84 

15.0 

11.0 

15.0 

18.0 

11.0 

3130 

22.0 
22.0 

16.0 
15.8 

1.38 

55 

78.0 

/22.0X 
\22.oJ 

/40.2X 
V39.8; 

.54 

26.4 

33.2 

22.6 

41.7 

18.0 

12.8 

1.40 

/18.0X 

/34.4X 

.51 

17.8 

12.8 

\l7-8/ 

\34-V 

18.2 

35.8 

2804 

(173) 

(127) 

1.44 

55 

80.0 

(173) 

(318) 

.43 

20.0 

25.2 

215 

149 

215 

503 

(324) 

(273) 

1.36 

(324)' 

(466) 

.40 

629 

464 

629 

550 

2653 

(173) 

(129) 

1.43 

55 

81.0 

(173) 

(388) 

.32 

14.0 

17.6 

215 

150 

215 

675 

(167) 

(124) 

1.42 

(167) 

(369) 

.33 

204 

144 

204 

622 

2536 

/63.2X 

/47.2X 

1.38 

55 

78.0 

/63.2X 

/213X 

.25 

8.4 

10.6 

\64.oJ 

\46.8y 

1.64.0/ 

\213j 

68.5 

49.5 

68.5 

278 

/65.0X 
V64.4J 

/48.6X 
\49.0) 

1.35 

/65.0X 
\64.4j 

(231) 

.22 

69.5 

51.5 

69.5 

309 

However,  line  2804  A.  has  a  very  low  intensity  and  perhaps  not  much 
importance  can  be  attached  to  this  value. 

A  second  set  of  sodium  and  potassium  mirrors  backed  by  the  same 
quartz  plates  gave  similar  curves  with  the  exception  of  the  minimum  of 
sodium,  which  occurs  slightly  farther  towards  the  short  wave-lengths. 


119  MABEL  K.  FREHAFER. 

No  other  change  was  made  in  the  apparatus  except  for  the  hole  in  the 
brass  support,  which  was  enlarged  to  8  mm. 

OBLIQUE  INCIDENCE:  POLARIZED  LIGHT. 

A  Rochon  prism,  one  part  of  which  was  quartz  and  the  other  part 
calcite,  cemented  with  glycerine,  was  used  as  polarizer.  In  the  path  of 
the  light  from  Sz  was  placed  a  quartz  lens,  following  this  the  Rochon 
prism^  finally  the  mirror  at  the  focus  of  the  lens  in  such  a  position  as  to 
receive  only  one  image.  The  angle  of  incidence  used  was  45°.  Since 
the  quartz  plates  backing  the  metals  are  fused,  and  nothing  is  interposed 
between  the  Rochon  prism  and  the  mirror,  the  state  of  polarization  of 
the  light  is  known  as  the  metal-quartz  surface.  The  ordinary  ray  was 
used,  with  the  electric  vector  either  in  the  plane  of  incidence  or  per- 
pendicular to  it,  effected  by  turning  the  prism  through  90°.  The  extra- 
ordinary ray  fell  beyond  the  mirror  on  the  soot-blackened  surface  of  the 
brass  support.  The  intensity  of  the  ordinary  ray  when  polarized  so  that 
the  electric  vector  is  in  the  plane  of  incidence  was  not  the  same  as  when 
it  was  polarized  so  that  the  electric  vector  is  perpendicular  to  the  plane 
of  incidence,  except  for  two  of  the  lines  used.  The  ratio  of  the  two  was 
found  by  removing  the  brass  support  and  placing  the  photoelectric  cell 
in  the  direct  beam  of  light.  Two  methods  were  used  (see  Fig.  6) ;  that 


Fig.  6. 

of  focussing  the  light  just  within  the  window  of  the  cell,  so  that  the 
cell  receives  practically  the  whole  beam  of  light;  and  that  of  placing  the 
window  of  the  cell  in  the  diverging  beam  so  that  the  central  part  of  the 
image  is  on  the  window.  The  two  methods  agreed  within  5  per  cent., 
which  is  as  accurate  as  could  be  expected.  Because  of  deficiencies  of  the 
Rochon  prism,  the  image  shifts  sidewise  and  downward  when  the  prism 
is  turned ;  the  mirror  (8  mm.  diameter)  is  sufficiently  large  to  accommo- 
date this  shift,  but  the  reflected  image  travels  away  from  the  window  of 
the  cell.  Hence  it  became  necessary  to  adjust  the  cell  for  each  adjust- 
ment of  the  Rochon  prism.  Lines  3341  A.  and  3130  A.  lose  greatly  in 
intensity  on  passage  through  the  prism ;  hence  the  difficulty  of  setting 
the  cell  accurately  for  the  reflected  images  of  these  lines  became  very 
great.  A  large  number  of  observations  have  been  made,  but  not  much 
accuracy  can  be  claimed  for  the  values  obtained.  An  error  of  10  per 
cent,  or  even  more  is  highly  probable. 


VOL.  XV.l 
No.  2. 


ULTRA-VIOLET  LIGHT. 


I  2O 


The  wave-lengths  below  3130  A.  were  absorbed  by  the  prism. 

Whether  the  cell  is  used  for  the  direct  beam  or  the  reflected  beam, 
the  direction  of  the  electric  vector  relative  to  the  sodium  surface  of  the 
cell  remains  the  same,  because  of  the  spherical  shape  of  the  cell. 

The  ratio  of  the  intensities  of  the  reflected  beams  E\\/E±  was  investi- 
gated as  a  function  of  the  wave-length.  The  sodium  or  potassium  mirror 
having  been  adjusted  in  the  brass  support  and  the  spectrograph  set  for 
a  given  wave-length,  the  rate  of  leak  of  the  electroscope  was  found  for 
each  component.  This  ratio,  when  plotted  against  the  wave-lengths, 
should  yield  the  information  desired  in  relation  to  the  photoelectric 
effect,  if  any  marked  change  in  the  reflection  occurs :  that  is,  such  a  curve 
would  correspond  to  the  En/E^-photoelectric  current  curve  referred  to 
on  page  2? 

Some  of  the  data  are  shown  in  Table  II.  The  first  numbers  in  the 
seventh  column  are  obtained  by  taking  the  inverse  ratio  of  the  rates  of 
leak  in  columns  two  and  three;  and  similarly  for  the  eighth,  fourth  and 
fifth  columns. 

TABLE  II. 


Wave 
Length. 

Polarized  Light.     Angle  of  Incidence  45°. 

Time  in  Seconds. 

Ratio  of  Intensities  EH/E±. 

Na. 

K. 

Incident 
Light. 

Reflected  Light 
Na. 

Reflected  Light 
K. 

4» 

B» 

*. 

M* 

5461 

29 

24 

17.0 

12.6 

.91 

.834-    .91=    .91 

.744-  .91=  .81 

30 

25 

17.2 

12.6 



4358 

10.6 

9.6 

21.6 

17 

.93 

.904-  .93=   .97 

.794-  .93=  .85 

10.8 

9.6 

4047 

9.6 

8.8 

27 

24 

1.03 

.924-1.03=  .89 

.9  4-1.03=  .88 

9.8 

9.0 

26 

24 

3650 

7.6 

9.2 

18 

19.4 

1.0 

1.214-1       =1.21 

1.094-1      =1.09 

17.8 

19.6 

3341 

(49) 
(50) 
56 

(38) 
(37) 
41 

29.8 
30 

23 
23.6 

1.0 

.734-1       =   .73 

.784-1       =  .78 

3130 

17 

14.2 

15 

10 

.77 

.824-  .77  =  1.06 

.684-  .77=  .88 

17.2 

14 

14.6 

10 

The  graphs  of  Fig.  7  represent  the  average  of  a  number  of  observations. 
The  maximum  in  the  neighborhood  of  3650  A.,  common  to  both  metals, 
is  rather  striking,  but  undoubtedly  is  due  to  the  metals  and  not  the 
quartz,  the  reflection  of  which  for  unpolarized  light  is  less  than  6  per 
cent,  at  45°.  It  cannot  be  due  to  a  "  selective  effect  "  from  the  cathode 
of  the  cell,  for  the  reason  given  above.  The  minimum  for  sodium  in  the 
neighborhood  of  3341  A.  together  with  the  maximum  at  3650  A.  shows  a 


121 


MABEL  K.  FREHAFER. 


("SECOND 

[SERIES. 


decided  change  in  the  reflecting  power  for  the  two  beams  polarized  at 
right  angles,  through  the  range  of  wave-lengths  in  which  the  maximum 
of  the  selective  effect  occurs.  When  the  electric  vector  is  so  directed 
that  it  has  a  component  perpendicular  to  the  illuminated  surface  of  the 
metal,  it  appears  that  at  the  critical  wave-length  of  the  selective  effect 
less  light  energy  is  reflected  by  the  metal,  hence  more  is  absorbed.  This 


RETLECTION 


;L_F 


5500  4000 

WAVE.  LENGTH 


or  F'OJLAKIZILD 


odi 


LKiH" 


Fig.  7. 

is  not  what  might  have  been  expected  from  the  usual  point  of  view,  that 
the  selective  effect  is  due  to  a  large  absorption  of  energy  near  the  surface — 
that  is,  the  extinction  coefficient  becomes  large,  from  which  a  higher 
reflecting  power  is  expected.  According  to  Nathanson's  data  the  ratio 
E\\/E±  is  less  than  unity  in  the  visible  spectrum  for  45°  incidence,  al- 
though his  value  for  potassium  is  considerably  higher  than  the  one  given 
here. 

Potassium  again  fails  to  show  any  marked  deviation  at  4358  A.,  but 
this  may  possibly  be  due  to  the  absence  of  data  in  the  interval  4358- 
5461  A.  Without  this  corresponding  minimum  for  potassium  it  is 
hardly  possible  to  draw  any  conclusions  as  to  whether  there  is  or  is  not  a 
direct  connection  between  the  reflecting  powers  of  the  metals  and  the 
selective  effect.  This  work  must  be  regarded  only  as  a  preliminary 
attempt. 

FILMS  DEPOSITED  AT  Low  TEMPERATURES. 

An  attempt  has  also  been  made  to  get  the  reflecting  power  of  potassium, 
and  the  transmitting  power  of  potassium  and  sodium,  using  films  of  the 
metals  deposited  on  quartz  plates  and  glass  plates  at  low  temperatures. 
The  apparatus  for  the  reflection  is  represented  in  Fig.  8.  Bulb  A  is 


NoU2V"]  ULTRA-VIOLET   LIGHT.  122 

coated  with  potassium  which  has  been  distilled  over  from  tubes  similar 
to  those  shown  in  Fig.  2.  In  the  bottom  of  this  bulb  is  a  piece  of  glass 
half  of  which  is  plated  with  nickel  (deposited  by  cathode  sputtering). 
This  half  is  covered  by  a  piece  of  metal  C  which  rests  on  M,  but  can 
readily  be  shaken  off.  On  the  lower  side  of  M  is  attached  a  piece  of 
iron,  by  means  of  which  M  can  be  manipulated  with  a  magnet.  In  the 
bulb  B  is  a  piece  of  brass  with  a  groove  in  which  M  can  slide  back  and 
forth.  Q  is  a  quartz  window.  The  lower  half  of  B  and  the  lower  part 
of  A  are  placed  in  vessels  containing  liquid  air.  When  sufficient  time 


has  elapsed  to  allow  the  cooling  to  take  place,  by  means  of  a  small  flame 
the  potassium  is  driven  down  from  the  top  of  A  on  to  the  uncovered 
part  of  M.  Opaque  films  only  were  used.  M  is  now  brought  quickly 
over  into  B,  which  is  kept  in  liquid  air  throughout  this  experiment. 

The  light  from  S%  (see  Fig.  8)  was  passed  through  a  quartz  lens,  then 
by  means  of  a  right-angled  quartz  prism  was  directed  downward,  almost 
vertically,  through  Q  on  to  M,  which  is  now  in  the  groove  in  B. 
By  means  of  a  magnet  the  nickel  surface  and  the  potassium  surface  were 
in  turn  brought  into  the  center  of  the  groove,  and  reflected  back  the 
light  through  Q,  into  the  photoelectric  cell.  In  this  case  nickel  served 
as  the  comparison  mirror.  The  values  of  the  reflecting  power  of  nickel 
are  taken  from  those  given  by  Hagen  and  Rubens1  in  the  visible  spec- 
trum, and  by  Hulburt11  in  the  ultra-violet  spectrum.  The  reflecting 
powers  of  the  two  metals  are  approximately  inversely  proportional  to 
the  rates  of  leak  of  the  electroscope.  Due  to  the  presence  of  gases  or 
impurities  on  the  glass  M  these  mirrors  were  somewhat  cloudy.  The 
reflection  curves  in  general  follow  the  one  shown  for  potassium  in  Fig.  5, 
but  the  values  are  somewhat  low. 

For  the  transmitting  power,  a  form  of  apparatus  such  as  is  shown  in 
Fig.  9  was  used.  A  glass  tube  with  two  quartz  windows  Qi,  Qz  contained 
a  metal  partition  P  with  a  hole  H.  The  part  of  the  tube  at  P  was  kept 

1  Hagen  and  Rubens,  Annalen  der  Physik,  i,  352,  1900. 


123 


MABEL  K.  FREHAFER. 


[SECOND 

L  SERIES  . 


cold  by  means  of  cotton  batting  soaked  with  liquid  air.  Part  of  bulb  A , 
containing  the  quartz  plate  Q,  was  cooled  in  liquid  air,  and  as  before  the 
potassium  was  driven  on  to  Q  by  heating  A  gently  with  a  small  flame. 
(Bulb  B  contains  the  sodium.)  Q  was  then  brought  quickly  into  the 
main  tube  up  against  P  so  that  H  was  completely  covered.  The  tube 


o 


Q 

P 

Q* 

\ 

H 

E 

S. 

r—  -1 

r~ 

e> 

Fig.  9. 

was  so  supported  that  it  could  readily  be  moved  into  the  beam  of  light 
or  out  of  it.  Thus  the  cell  received  in  turn  the  transmitted  beam  and 
the  direct  beam.  It  is  also  a  simple  matter  to  remove  the  film  of  metal 
from  Q  and  to  measure  the  loss  of  intensity  of  the  incident  light  due  to 

no*- 


60 


040 


r 

IG. 

10 

T 

RA 

NSF 

rtlS 

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Sa 

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50 

=-= 
=^=i 

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1  —  -- 

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iOO               3C 

00'               3500                 4000                4500 

LENGTH. 


Fig.  10. 

Qi,  Q2  and  Q;  and  by  removing  <2  the  loss  of  intensity  due  to  Qi  and  Q2. 
The  transmitting  power  of  the  film  is  then  readily  computed. 

The  results  are  shown  for  a  few  films  in  the  graphs  of  Fig.  10.     The 


NoL  2XV']  ULTRA-VIOLET   LIGHT. 

divergence  of  the  curves  for  the  metallic  and  colloidal  films  is  noticeable, 
particularly  in  the  visible  region.  The  most  striking  fact  is  the  greater 
transmitting  power  of  potassium. 

SUMMARY. 

1.  The  reflecting  powers  of  sodium  and  potassium  surfaces  backed  by 
quartz  have  been  determined  directly  for  normal  incidence  (10°),  as  a 
function  of  the  wave-length,  for  the  range  of  wave-lengths  5461  A.- 
2536  A.     Sodium  is  the  best  reflector  known  in  the  ultra-violet  region, 
giving  80  per  cent,  at  2536  A.;   potassium  is  the -worst  reflector  in  the 
ultra-violet,  giving  n  per  cent,  at  2536  A.     In  the  case  of  sodium,  there 
is  evidence  of  a  flat  minimum  in  the  neighborhood  of  3300  A-36oo  A. 

2.  The  reflecting  power  of  these  surfaces  for  polarized  light  has  been 
measured  in  the  region  5461  A.-3I3O  A.  by  a  determination  of  the  ratio 
E\\/E±  as  a  function  of  the  wave-length  (E\\  is  the  intensity  of  the  reflected 
light  when  the  electric  vector  of  the  incident  light  is  parallel  to  the  plane 
of  incidence,  E±  the  intensity  of  the  reflected  light  when  the  electri- 
vector  of  the  incident  light  is  perpendicular  to  the  plane  of  incidence); 
Both  metals  show  a  maximum  value  of  this  ratio  in  the  vicinity  of  3650  A.. 
sodium  shows  a  decided  minimum  in  the  vicinity  of  3341  A. 

3.  Metallic  films  of  the  two  metals  have  been  deposited  at  low  temperac 
tures,  and  the  reflecting  and  transmitting  powers  of  these  films  examined. 
This  promises  to  be  a  fruitful  field  of  research,  .as  the  photoelectric 
current  and  optical  properties  can  be  examined  as  a  function  of  the 
thickness  of  the  film. 

The  transmitting  power  of  potassium  is  decidedly  higher  than  that  of 
sodium. 

It  gives  me  great  pleasure  to  express  my  gratitude  to  Professor  Ames 
for  his  interest  throughout  the  course  of  this  work,  and  to  Professor 
Pfund,  who  proposed  the  problems  taken  up,  and  whose  assistance  and 
helpful  suggestions  have  made  the  execution  of  the  work  possible. 

BIOGRAPHICAL  NOTE. 

Mabel  Katherine  Frehafer,  daughter  of  Charles  Milton  Frehafer  and 
Caroline  (Ball)  Frehafer,  was  born  in  Philadelphia  on  July  7,  1886.  Her 
early  training  was  received  in  Philadelphia.  In  1904  she  entered  Bryn 
Mawr  College,  and  in  1908  received  the  degree  of  Bachelor  of  Arts.  She 
held  a  scholarship  at  Bryn  Mawr  from  the  City  of  Philadelphia  for  the 
years  1904-8.  During  the  year  1908-9  she  was  a  graduate  student  in 
physics  and  mathematics  at  the  University  of  Wisconsin,  and  in  1909 
received  the  degree  of  Master  of  Arts.  She  was  appointed  fellow  in 


1 2*5  MABEL  K.  FREHAFER.  [SECOND 

<J  LSERTE  S. 

physics  at  Bryn  Mawr  College  for  the  year  1909-10.  From  1910-14 
she  was  demonstrator  in  physics  at  Bryn  Mawr  College.  She  was  en- 
rolled as  a  student  of  physics  and  mathematics  in  the  summer  school  of 
Wisconsin  in  1914.  During  the  year  1914-15  she  was  half  time  assistant 
and  student  at  the  University  of  Wisconsin.  For  the  first  semester  of 
1915-16  she  was  full  time  assistant  at  Wisconsin,  and  for  the  second 
semester  teacher  of  physics  in  the  Germantown  High  School  for  girls, 
Philadelphia.  For  two  years  beginning  1916  she  was  instructor  in 
physics  at  Mt.  Holyoke  College.  She  entered  Johns  Hopkins  Uni- 
versity as  a  graduate  student  in  October,  1918,  making  physics  the 
principal  subject,  mathematics  the  first  subordinate,  and  chemistry  the 
second  subordinate.  She  attended  courses  in  physics  and  mathematics 
given  by  Professor  Ames  and  Dr.  Murnaghan. 


THE  AMERICAN  PHYSICAL  SOCIETY 


OFFICERS  OF  THE  SOCIETY 

President:  J.  S.  AMES  Johns  Hopkins  University,  Baltimore,  Md. 

Vice  President:         THEODORE  LYMAN  Harvard  University,  Cambridge,  Mass. 

Secretary:  D.   C.   MILLER  Case  School  of  Applied  Science,  Cleveland,  Ohio 

Treasurer:  G.  B.  PEGRAM  Columbia  University,  New  York  City 

Editor:  F.   BEDELL  Cornell  University,  Ithaca,  N.  Y. 

Local  Secretary  for 

the  Pacifis.  Coast:      E.  P.  LEWIS  University  of  California,  Berkeley,  Cal. 

The  Council  of  the  Society  consists  of  the  president,  vice-president,  secretary,  treasurer,  manag- 

i! or,  all  living  Past  Presidents  and  eight  elected  members  as  follows: 
Past  Presidents:  A.  A.  Michelson,  A.  G.  Webster,  Carl  Barus,  E.  L.  Nichols,  Henry  Crew, 

W.  F.  Magie,  Ernest  Merritt,  R.  A.  Millikan,  H.  A.  Bumstead. 
Elected  Mimbers-:  G  O.  Squier,  H.  A.  Wilson,  A.  L.  Day,  G.  F.  Hull,  G.  K.  Burgess,  J.  C. 

McLennan,  J.  B.  Jewett,  Max  Mason. 
A  complete  list  of  past  officers  is  given  on  a  following  page. 


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